1. Field of the Invention
The present invention relates to a semiconductor integrated circuit. More particularly, the present invention relates to a semiconductor integrated circuit which comprises a drive control signal output circuit for driving a display element and a display apparatus which comprises the display element.
2. Description of the Prior Art
FIG. 12 is a circuitry diagram showing connections between a light emitting diode array 2 and a conventional semiconductor integrated circuit 200 which drives the same. The light emitting diode array 2 is comprised of eight light emitting diodes 3a, 3b, . . . , 3h. Cathode electrodes of the light emitting diodes are commonly connected to a cathode of a drive power source 6 at a terminal 4. Anode electrodes of the light emitting diodes 3a, 3b, . . . , 3h are independent of each other and are respectively connected to drive signal output terminals 5a, 5b, . . . , 5h of the semiconductor integrated circuit 200. A drive power source input terminal 7 of the semiconductor integrated circuit 200 is connected to an anode of the drive power source 6.
FIG. 13 is a block diagram showing an inner structure of a chip 8 of the semiconductor integrated circuit 200. The chip 8 includes drive signal output pads 10a, 10b, . . . , 10h and a drive power source input pad 14 which are respectively connected to the drive signal output terminals 5a, 5b, . . . , 5h and the drive power source input terminal 7 within the semiconductor integrated circuit 200.
The chip 8 also includes a drive control signal generation circuit 13 and drive circuits 9a, 9b, . . . , 9h. The drive control signal generation circuit 13 outputs drive control signals 11a, 11b, . . . , 11h which respectively control the drive circuits 9a, 9b, . . . , 9h. From the power source input pad 14 on a drive power source line 15, potentials are supplied to the drive circuits 9a, 9b, . . . , 9h respectively at drive circuit input terminals 12a, 12b, . . . , 12h, and the drive circuits 9a, 9b, . . . , 9h transmit the potentials to the drive signal output pads 10a, 10b, . . . , 10h, respectively.
The light emitting diodes 3a, 3b, . . . , 3h of the light emitting diode array 2 each emit light in accordance with the drive control signals 11a, 11b, . . . , 11h as shown in FIGS. 12 and 13.
It is to be noted here that the drive power source line 15 has a resistance component as a wire resistance. Hence, drive output impedances of the drive signal output pads 10a, 10b, . . . , 10h are each equal to a sum of the ON-resistance of each of the drive circuits 9a, 9b, . . . , 9h and the wire resistance the drive power source line 15 has within the chip 8.
FIG. 14 is a block diagram showing the resistance component the drive power source line 15 has within the chip 8. The drive power source line 15 includes a wire resistance 19a at a portion thereof which is located between the drive power source input pad 14 and the drive circuit input terminal 12a of the drive circuit 9a which is disposed closest to the drive power source input pad 14, and a wire resistance 19b at a portion which is located thereof between the drive circuit input terminal 12a of the drive circuit 9a and the drive circuit input terminal 12b of the drive circuit 9b. In this manner, wire resistances 19c, 19d, . . . 19h are disposed.
If the drive output circuits 9e and 9f have the same ON-resistance, a difference in drive impedance between the drive signal output pads 10e and 10f, for instance, which are seated adjacent each other within the chip 8 is equal to the resistance value of the wire resistance 19f which exists on the drive power source line 15 at a portion between the drive circuit input terminals 12e and 12f. Since a difference in drive impedance between any adjacent drive signal output pads is not large and drive output impedance differences among any adjacent pairs of the drive signal output pads are approximately the same, there is no chance that the display state of the diode array 2 will be extremely unnatural.
However, even within the same chip 8, the closest drive signal output pad to the drive power input pad 14 has the lowest drive output impedance, and progressively farther drive signal output pads have progressively higher drive output impedances. More precisely, the wire resistances 19a, 19b, . . . , 19h are formed in series between the drive power source input pad 14 and the drive circuit input terminal 12h of the farthest drive signal output pad 9h from the drive power source input pad 14. Due to this, if the drive output circuits 9a and 9h have the same ON-resistance within the semiconductor integrated circuit 200, the drive output impedance with respect to the drive signal output terminal 5h is larger than the drive output impedance with respect to the drive signal output terminal 5a by a sum of the resistance values of the wire resistances 19b, . . . , 19h. As a result, a large difference in brightness is created between the brightest light emitting diode 3a and the darkest light emitting diode 3h, with the brightness of the interposing light emitting diodes gradually decreasing.
This is particularly disadvantageous where a plurality of the semiconductor integrated circuits 200 are used together with an increased number of diodes within the light emitting diode array in order to realize a finer display. FIG. 15 is a circuitry diagram showing connections between a light emitting diode array 291 which consists of sixteen light emitting diodes 3a, 3b, . . . , 3p and two semiconductor integrated circuits 200a and 200b which control the light emitting diode array 291. Each of the semiconductor integrated circuits 200a and 200b includes the chip 8, the drive signal output terminals 5a, 5b, . . . , 5h and the drive power source input terminal 7 which are shown in FIG. 13, and operate similarly to the semiconductor integrated circuit 200.
The semiconductor integrated circuit 200b control eight light emitting diodes 3a to 3h while the semiconductor integrated circuit 200a control eight light emitting diodes 3i to 3p. If the light emitting diode array consists of a larger number of light emitting diodes, one or more additional drive integrated circuits are disposed depending on a need to drive the light emitting diode array.
The adjacent light emitting diodes 3h and 3i which are driven by different semiconductor integrated circuits are taken as an example. The light emitting diode 3h is connected to the drive output terminal 5h which has the highest drive output impedance within the semiconductor integrated circuit 200b, whereas the light emitting diode 3i is connected to the drive output terminal 5a which has the lowest drive output impedance within the semiconductor integrated circuit 200a. Hence, the brightness of the light emitting diode array 291 shows an abrupt change between these adjacent diodes, which is quite unnatural as a display.